1. Field of the Invention
This invention relates to apparatus for receiving broadcast signals, and more particularly, to a space diversity receiver which receives at least two signals and which choses the signal having the best signal to noise ratio to produce an output audio signal therefrom.
2. Description of the Prior Art
Diversity transmission is a form of transmission using several modes, usually in time or in space, to compensate for fading or outages in any one of the modes. In a space diversity system, the same signal is sent simultaneously over several different transmissions paths, which are separated enough so that independent propagation conditions can be expected. In diversity reception, the effects of fading during reception of a radio signal are minimized by combining or selecting two or more sources of a received signal which carry the same intelligence but differ in strength or signal to noise ratio.
A space diversity receiver of known construction for receiving broadcast signals includes a pair of receiving circuit blocks. Each receiving circuit block has associated therewith an antenna for receiving the broadcast signal. The space diversity receiver compares the level of the two received signals, or the noise components contained in the signals, in order to select the signal which has the better signal to noise ratio. The receiver uses the comparison to switch from one signal to the other as the strengths of the two signals change and produces an audio output signal from the stronger signal. However, objectionable pulse noise can be generated from the switching operation and this can be included in the audio output signal. This is especially noticeable when a space diversity receiver is used in an automobile since the location of the receiver changes over a relatively broad area and the intensity of the electric field of the broadcast signal varies quite frequently, so that frequent switching between the receiving circuit blocks occurs. The pulse noise is particularly conspicuous or objectionable when the intensity of the electric field of the broadcast signal is relatively weak. It is to be noted that the signals derived from the receiving circuits in such an instance have an inherently low signal to noise ratio, anyway, and produce a low quality audio output signal even without the addition of the pulse noise.
In order to reduce the pulse noise caused by switching between the signals from the receiving circuits, it has been proposed to supply the audio output signal to a hold circuit which "holds" or repeats the preceding level. Each segment of the audio output signal obtained during switching is replaced with the level of the immediately preceding signal segment so that the pulse noise is eliminated. However, in such a proposal, the high frequency components of the audio output signal are lost along with the pulse noise in the substituted portions of the signal. Accordingly, the quality of the audio output signal suffers significant degradation.